Abstract. Understanding the exchange processes between the atmospheric boundary layer
and the free troposphere is crucial for estimating hemispheric transport of
air pollution. Most studies of hemispheric air pollution transport have
taken a large-scale perspective using global chemical transport models with
fairly coarse spatial and temporal resolutions. In support of United Nations
Task Force on Hemispheric Transport of Air Pollution (TF HTAP;
www.htap.org), this study employs two high-resolution atmospheric chemistry
models (WRF-Chem and CMAQ; 36×36 km) driven with chemical boundary
conditions from a global model (MOZART; 1.9×1.9°) to examine the role of
fine-scale transport and chemistry processes in controlling pollution export
and import over the Asian continent in spring (March 2001). Our analysis
indicates the importance of rapid venting through deep convection that
develops along the leading edge of frontal system convergence bands, which
are not adequately resolved in either of two global models compared with
TRACE-P aircraft observations during a frontal event. Both regional model
simulations and observations show that frontal outflows of CO, O3 and
PAN can extend to the upper troposphere (6–9 km). Pollution plumes in the
global MOZART model are typically diluted and insufficiently lofted to
higher altitudes where they can undergo more efficient transport in stronger
winds. We use sensitivity simulations that perturb chemical boundary
conditions in the CMAQ regional model to estimate that the O3
production over East Asia (EA) driven by PAN decomposition contributes
20% of the spatial averaged total O3 response to European (EU)
emission perturbations in March, and occasionally contributes approximately
50% of the total O3 response in subsiding plumes at mountain
observatories (at approximately 2 km altitude). The response to decomposing
PAN of EU origin is strongly affected by the O3 formation chemical
regimes, which vary with the model chemical mechanism and NOx/VOC
emissions. Our high-resolution models demonstrate a large spatial
variability (by up to a factor of 6) in the response of local O3 to 20% reductions in EU
anthropogenic O3 precursor emissions. The response in the highly populated
Asian megacities is 40–50% lower in our high-resolution models than the global model,
suggesting that the source-receptor relationships inferred from
the global coarse-resolution models likely overestimate health impacts associated with
intercontinental O3 transport. Our results highlight the important
roles of rapid convective transport, orographic forcing, urban
photochemistry and heterogeneous boundary layer processes in controlling
intercontinental transport; these processes may not be well resolved in the
large-scale models.